#include <cstdio>

int main( )
{
    int n, w, d, s, r;
    while ( scanf("%d%d%d%d", &n, &w, &d, &s) != EOF )
    {
        r = ( n * ( n - 1 ) / 2 * w - s ) / d;
        printf("%d\n", r + ( r == 0 ) * n );
    }
    return 0;
}
